Thermal conductivity is a material property; board temperature is a system result
There is no single thermal-conductivity value for a “ceramic PCB.” Alumina, aluminum nitride and silicon nitride have different properties, and the result for a particular grade depends on composition and test temperature. A patterned circuit board also includes metal, attach layers and interfaces. Use a named grade’s datasheet for a calculation and keep material properties separate from the thermal resistance of the finished heat path.
How to read a ceramic thermal-conductivity number
Thermal conductivity, k, is reported in W/(m·K). A higher value means more heat can pass through the same geometry under the same temperature difference in the direction being measured. It does not say how much heat a particular board will dissipate to ambient air. Supplier tables illustrate why grade matters: Kyocera’s ceramic material data distinguish material grades, while CoorsTek’s alumina catalog shows properties for specified formulations. These are supplier examples, not AluminaPCB material certificates or guaranteed values.
When comparing data, record the grade, purity or formulation, density if supplied, temperature, direction and test method. Check whether the value describes bare ceramic or a metallized substrate. A value quoted without those conditions is useful only as an early material shortlist.
From conductivity to through-thickness resistance
For steady, one-dimensional heat flow through a uniform flat slab, the ceramic-only thermal resistance is Rceramic = t/(kA). Thickness t is in metres, conductivity k in W/(m·K), and heat-flow area A in square metres; resistance is in K/W. For example, a 0.5 mm slab with k = 25 W/(m·K) and an assumed 10 mm × 10 mm uniform heat-flow area gives 0.20 K/W. That is an illustrative calculation, not a prediction for a finished board. Real heat sources spread laterally; the effective area changes with the stack and geometry. NIST’s thermal measurement paper defines the one-dimensional terms.
Under the same simplified assumptions, doubling ceramic thickness doubles its through-thickness resistance; doubling the assumed heat-flow area halves it. The assembly adds die attach, conductor, interfaces, baseplate or heat sink. A high-conductivity substrate cannot compensate for an uncontrolled contact interface by itself. Rogers discusses the interaction of ceramic thickness and isolation in metallized substrate applications.
What changes when metal is added?
Metal on a ceramic substrate can spread heat laterally before it crosses the ceramic. It also changes where heat enters the substrate and may add process-specific reliability constraints. The material conductivity printed on a ceramic datasheet remains a property of the ceramic; it should not be replaced with an invented “effective conductivity” for every board. If the die, metal pattern, ceramic, attachment and cooling boundary are known, evaluate the complete heat path with an appropriate model or measurement. See the DBC versus DPC comparison for how metallization choices change the conductor structure.
Questions for a thermal design review
- What is the maximum heat load and where does it enter the substrate?
- What is the named ceramic grade, its datasheet conductivity and the temperature at which that value applies?
- What are the ceramic thickness, conductor geometry and actual contact area?
- Which attachment and interface materials are in the path, and how are they applied?
- What temperature limit applies at the die, joint or another critical location, and under what ambient and cooling conditions?
For material-family trade-offs, start with our ceramic material guide or the focused AlN versus alumina comparison. Those pages should be checked against a current grade datasheet before design release. If you have a drawing and heat-load estimate, send the design for a feasibility discussion and state the thermal assumptions you want reviewed.